Building construction project will not be separated from the cause of uncertainty and risk that will affect the optimization of the cost of construction projects. To make an estimation, one should be consider to varieties of factors that give influence, including the risk factors that in this study in terms of risk based on the characteristics of the project location. Estimated offer was made as accurately as possible because it affects the success of a construction project. So, it cause the need of analyzing the cost of uncertainty of the construction project.

Risks in construction projects based on the characteristics of project location consists of weather, remoteness of location, location limitations, availability of labors, the terrain, also security and vulnerability of location that cause uncertainty on the cost of each item of job. Risk factors are then carried out its frequency and impact of risk assessment through questionnaires to 13 contractors which were processed using a risk matrix, as well as analysis and risk factors weighting to determine the level of importance and the risk score using Analytical Hierarchy Process (AHP) method.

The items of work are taken from Budget Plan of Trivium Terrace Apartment project, which will be used to make a work breakdown structure and cost breakdown structure to determine the integration between the work and its cost. Pareto analysis was then performed to obtain important work items that 20% believed as the most important job of all items of work, which gives a value of 80% of the total project cost.

Pareto analysis along with the results of the risk assessment is used as a basis for interviews to determine the probability distribution function (PDF) of the uncertainty of unit price, in the form of pessimistic and optimistic values of unit price, due to the uncertainty of existing cost based on the experience of the expert from the contractor, while the volume is fixed. The results of the interview then used as inputs in the monte carlo simulation, that in this study is using Oracle Crystal Ball software.

From the simulation outputs, it can be concluded that the total cost of the Trivium Terrace Apartment project with confidence level of 50% amounting to Rp67, 229,090,525.92, with probability level not achieved or is likely to exceed the total cost amounted by 50% also. If the confidence level is taken to be 90%, the total cost amounting to Rp67, 478,998,764.04 with a potential uncertainty of this cost has the opportunity to not achieved or are likely to exceed the total cost amounted to only 10%, but it has a 90% probability of the project is carried out succeed with the total cost amounted.